But not everyone wants to do the same thing or be treated the same way. In fact, people all want to do different things. You can't get 100% of people to agree on anything. If you walked down the street with a hammer and asked 100 people "do you want to be hit in the head?" you couldn't get them all to say "no".

There are other objections that are more specific:

One of the first major challenges to Kant's reasoning came from the French philosopher Benjamin Constant, who asserted that since truth telling must be universal, according to Kant's theories, one must (if asked) tell a known murderer the location of his prey.

In this reply, Kant agreed with Constant's inference, that from Kant's own premises one must infer a moral duty not to lie to a murderer.

Kant denied that such an inference indicates any weakness in his premises: not lying to the murderer is required because moral actions do not derive their worth from the expected consequences.

https://en.wikipedia.org/wiki/Categorical_imperative

Basically, the categorical imperative is too inflexible to be practical.